1) Field of the Invention
This invention relates to a composition having a base of vinylidene polyfluoride (PVDF), an alkyl polymethacrylate and an acrylic and/or methacrylic elastomer coextrudable with the PVDF, which permits the adhesion of the latter to a polymeric resin incompatible therewith. The invention also relates to the coextrusion products of the composition with the PVDF and the compounds directly obtained by coextrusion of PVDF and thermoplastic resin incompatible with the PVDF, the composition serving as an adhesion agent.
2) Description of the Related Art
P.D. Frayer in 34th Annual Tech. Conference SPE (1976), Vol. 22, pages 89-90, points out that two incompatible polymers can adhere one to the other when coextruded with a copolymeric agent compatible with each one of them. If Frayer provides the concept of coextrusion to make non-compatible polymers adhere to each other, he does not define which polymers allows said adhesion. Everyone knows that fluorinated resins, and more particularly the PVDF, are not especially compatible with other thermoplastic resins and that it is not sufficient to say that the coextrusion of a fluorinated resin with a third compatible component allows a convenient adhesion with said thermoplastic resins. In FR 2,436,676, there is described a process of manufacture of a composite material PVDF-thermoplastic polymer non-compatible with the PVDF consisting in a coextrusion of the two components with an alkyl polymethacrylate or a mixture of at least 75% by weight of an alkyl polymethacrylate with another undefined polymer as the agent. It has been observed that even though said technique effectively allows one to obtain, by virtue of the PVDF, a good surface protection of the layers of thermoplastic resins, their resistance to shock and their long-range firmness with respect to humidity are mediocre.
By way of example, a layer of acrylonitrile-butadiene-styrene resin coated with a PVDF film obtained according to the method of Example 1 of FR 2,436,676 with a film of methyl polymethacrylate as the agent initially shows an excellent interlayer adhesion above 2500 N/m. This adhesion becomes poor: 480 N/m after a 400-hour exposure at 75.degree. C. in an atmosphere saturated with water. Besides, the resistance to shock is only on the order of from 300 to 380 Kj/m.sup.2. Even though methyl polymethacrylate is known for its compatibility with PVDF and other polymers, it appears that this compatibility according to Frayer, if it is necessary, is not sufficient to define a good adhesion.